The present invention relates to cathode-ray tube display terminals and is directed more particularly to a display terminal which is adapted to present character information, in any one of a plurality of different formats, within a display frame having approximately constant dimensions.
In systems in which a host computer operates in conjunction with a plurality of display terminals, with or without an intermediate controller, it is common for the host computer to run application programs that instruct each terminal to display character information in a particular logical screen size or display format. The host computer may, for example, request the terminal to present character information in a 1920 screen size which includes 24 character rows of 80 characters each plus one 80 character status row, or in a 2560 screen size which includes 32 character rows of 80 characters each plus one 80 character status row, etc. Prior to the present invention, terminals were designed to present character information in only one format, which format might or might not match that requested by the host computer.
In the event that a terminal of the above-mentioned type is requested to use a logical screen size other than the one for which it was designed, one of two results will occur. Firstly, if the logical screen size is smaller than the physical screen size, characters will be displayed within a display frame which includes an unsightly blank area having a size which is proportional to the difference between the physical and logical screen sizes. Secondly, if the logical screen size is larger than the physical screen size, the display will be forced to reject the host computer's request to use that logical screen size. In order to avoid these results, it has been necessary for an operator to move to or connect up a terminal which was designed to use the desired screen size.
Prior to the present invention, there were a number of reasons why terminals were designed to use only a single display format. One reason is that changes in the number of character rows within a display frame require changes in the number of horizontal scan lines that must be presented during the vertical period of the display. In addition, changes in the number of character columns within a display frame require changes in the number of horizontal dots or pixels that must be presented during the horizontal period of the display. The problem of dealing with these different numbers of lines and pixels is compounded by the fact that both of these variables impact the refresh rate of the display, which must be maintained at a high enough rate to prevent the display from producing a perceptible flicker. This problem is further compounded by the fact that changes from one display format to another can cause the aspect ratios of the characters, i.e., the ratios of the widths to the heights of the characters, to vary outside of acceptable limits.
In order to solve the above-described problems, it had appeared, prior to the present invention, that the ability of a single terminal to accommodate a plurality of different display formats required the provision of a plurality of different selectable horizontal and vertical deflection circuits, a plurality of different character clock and dot clock signal generators, a plurality of different selectable character memories, and circuitry for controlling the selection thereof. Because the use of this approach requires the duplication of a substantial portion of the terminal circuitry, the cost of its adoption has been prohibitive.